This invention relates to ignition circuits.
High energy ignition systems employ a capacitor to store electrical energy which is then rapidly discharged to an igniter or spark plug to produce an intense spark sufficient to light a fuel-air mixture. A typical solid-state igniter may require up to 2000 volts to cause break-over. Once the spark has commenced, the igniter voltage collapses to near zero while a current of approximately 2000 amperes flows for the duration of the spark until the energy in the capacitor has dissipated. Normally this cycle of charging and discharging of the stored energy is repeated many times until satisfactory ignition of the fuel occurs.
Some high energy ignition systems employ a gas discharge tube, which breaks over at a point when the voltage on the charging storage capacitor reaches the desired level to xe2x80x98dumpxe2x80x99 the accumulated charge into the igniter. For various technical reasons including, life expectancy, synchronisation, mechanical robustness and reliability, it is desirable to use solid state electronic switching of the discharge; the most suitable component for this is a thyristor. At present, suitable devices are not cheaply available to handle 2000 volts directly and the high currents in this application. However, devices to switch comfortably at 1000 volts are easy to obtain and are relatively low cost.
It is an object of the present invention to provide an alternative ignition circuit.
According to one aspect of the present invention there is provided an ignition circuit including storage means to store electrical energy, first and second switching devices, means for charging the storage means, and means for turning on each of the switching devices so that charge on the storage means is transferred to cause firing of an igniter in such a way that the voltage across each switching device is limited to a fraction of the total applied to the igniter. This makes it possible to use relatively inexpensive switching devices, capable of handling a moderate voltage, whilst providing the igniter with a sufficiently high voltage to cause sparking.
Preferably, the storage means comprises a double storage means and provides a reduced voltage point, compared to the total voltage applied to the igniter, which limits the voltage applied across each switching device.
The storage means preferably includes two storage capacitors. The voltage across each switching device is preferably substantially half the total voltage applied to the igniter.